Mark of Ephesus
Our Father Among the Saints Mark of Ephesus (Evgenikos), Pillar of Orthodoxy, was famous for his courageous defense of Orthodoxy at the Council of Florence (1439 A.D.) in spite of the emperor and the pope of Rome. He held Rome to be heretical for its acceptance of the Filioque clause added to the Nicene-COnstantinopolitan Creed, and was thus the only Eastern bishop to refuse to sign the decrees of the council.
For his efforts at the Council of Florence and his defense against the addition of the Filioque, the Orthodox Church considers him a saint, calling him a Pillar of Orthodoxy. His feast day is January 19.
"It is impossible to recall peace without dissolving the cause of the schism—the primacy of the Pope exhalting himself equal to God." 
"The Latins are not only schismatics but heretics... we did not separate from them for any other reason other than the fact that they are heretics. This is precisely why we must not unite with them unless they dismiss the addition from the Creed filioque and confess the Creed as we do."